Excel 2007 : Excel 2007 formulation

Hi All,

I need help with this problem;


School Number of Students Number of Teachers Student/Teacher Ratio
Cleveland 741 43 17.23
Parkside 760 38 23.26
Powers 643 37 17.38
Lakeview 780 39 23.23
Sunnyslope 704 40 17.6
Praireview 715 41 17.44
Mountainview 793 40 19.83
Total 5136 278 19.37
of course it did not paste smoothly but, the idea is 741 is under # of students, 43 is under # of teachers, and 17.23 is under student/teacher ratio. I had no problem in calculating cleveland to mountainview, but when it came to the final ratio, I could not find the formula that would give me the final answer of 19.37, help please!!!!! I did an average of ratios and that gave me 19.42, what am I doing wrong??????????